This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Nitric oxide synthases (NOS) are a family of heme- and flavin-containing enzymes that catalyze the NO formation from L-arginine. NO produced by three different NOS isoforms in mammals has been implicated in a wide range of physiological functions as well as pathological conditions. Development of isoform-selective NOS inhibitors is of therapeutic importance in blocking the NO overproductions mainly by neuronal and/or inducible NOS under the pathological settings while maintaining the vascular regulatory role of NO generated by endothelial NOS. Crystal structures of three NOS isoforms revealed a highly resembled substrate-binding heme active site. The structural similarity of the NOS active site makes the search of isoform selective inhibitors a challenging task. Extensive structural characterizations of a variety of bound ligands are required before useful information can be extracted. The structural knowledge can then be used in design of new compounds that bear better inhibitory potency and/or isoform selectivity. Extension of this proposal is vital to the continuation of this research.